1. Field of the Invention
The present invention relates to a die assembly for molding optical disc substrates and, in particular, it relates to a die assembly for molding optical disc substrates that comprises an outer circumferential-stamper holder having: a stamper holding portion that holds an outer circumferential edge portion of a stamper disposed on a specular plate; and an inner circumferential portion that defines an outer circumferential end of a molding cavity.
2. Description of the Related Art
FIG. 5 and FIG. 6, which is an enlarged view of a part enclosed by a circle 6 in FIG. 5, show an example of a conventional die assembly 100 for molding an optical disc substrate Mp. This die assembly 100 is comprised of a stationary die 11 and a movable die 200. The stationary die 11 comprises: a stationary die plate 13 that is attached to a stationary platen (not shown) of an injection molding machine via a thermal insulation plate 14; a stationary specular plate 16 that is fastened to a surface of the stationary die plate 13, which is opposite to that for attaching the thermal insulation plate 14, via a stationary back plate 15; a gate insert 19 that are inserted through center openings of the stationary die plate 13, the stationary back plate 15 and the stationary specular plate 16; a sprue bush 18 and a positioning plate 17; and a stationary outer circumferential ring 12 that is inserted around outer circumferential end faces of the stationary back plate 15 and the stationary specular plate 16 and fastened to the stationary die plate 13.
The movable die 200 comprises: a movable die plate 22; a movable specular plate 24 that is fastened to a surface of the movable die plate 22 toward the stationary die 11 via a movable back plate 23; a stamper 26 that is disposed on a surface of the movable specular plate 24 faced by the stationary specular plate 16; an outer circumferential stamper holder 250 that pinches an outer circumferential edge portion of the stamper 26 to allow the movable specular plate 24 to hold the stamper 26; an inner circumferential stamper holder 28 that is movably inserted through center openings of the movable back plate 23 and the movable specular plate 24 and that pinches an inner circumferential end of the stamper 26 to allow the movable specular plate 24 to hold the stamper 26; a stationary sleeve 29 that guides an internal hole of the inner circumferential stamper holder 28 and that is fastened to the movable die plate 22; an ejector 30 that is inserted through an internal hole of the stationary sleeve 29 slidably in the axial direction; a male cutter 31 that is inserted through an internal hole of the ejector 30 slidably in the axial direction; an ejector pin 32 that is inserted through an internal hole of the male cutter 31 slidably in the axial direction; and a movable outer circumferential ring 21 that is inserted around outer circumferential end faces of the movable back plate 23 and the movable specular plate 24 and fastened to the movable die plate 22. As the stationary outer circumferential ring 12 and the movable outer circumferential ring 21 abut against each other, the movable die 200 is aligned with the stationary die 11 described above to constitute the die assembly 100 and form a molding cavity 270 therebetween.
As shown in the enlarged cross-sectional view of FIG. 6, the outer circumferential stamper holder 250 has: a stamper holding portion 340 that holds the outer circumferential edge portion 26E of the stamper 26 disposed on the movable specular plate 24; and an inner circumferential portion 330 that defines an outer circumferential end 270E of said molding cavity 270. Then, the inner circumferential portion 330 has a surface 330S that has a linear cross-section extending from a recessed portion 41 opposed to a protruding portion 40 of the stationary specular plate 16. In this connection, the inner circumferential surface 330S may have a relatively small step portion having a height and width of 0.1–0.5 mm, as described in Japanese Unexamined Patent Publication No. H08-132488. When the optical disc substrate Mp is molded in the molding cavity 270 of the die assembly 100 configured as described above, melted resin permeates through a clearance D of about 5–10 im between the protruding portion 40 and the recessed portion 41 to form a burr 36. The burr 36 may fall off in the process after the optical disc substrate Mp is molded to cause contamination or, further, it may obstruct printing on a reading surface that is opposite to a signal surface to which information is transferred by the stamper 26 for the optical disc or it may spoil a feeling when a user touches the optical disc by hand. This problem may be solved by reducing the clearance D of about 5–10 im between the protruding portion 40 and the recessed portion 41 or by reducing the inclination angle C of the protruding portion 40 and the recessed portion 41 but, in this case, another problem occurs in that galling is likely to occur between the protruding portion 40 and the recessed portion 41.
Further, as shown in FIG. 7, when an optical disc Mpo is manufactured by bonding the signal surfaces of such optical disc substrates Mp, Mp together, there is a problem in that an adhesive or UV lacquer G leaks through the interface between the bonded surfaces to spoil appearance. In order to address this problem, Japanese Unexamined Patent Publication No. H08-315430 discloses a method in which optical disc substrates are molded with step portions to solve this problem by the die assembly disclosed in Japanese Unexamined Patent Publication No. H08-132488 and, then, are bonded together. However, the method using such step portions cannot prevent the leakage of the UV lacquer G entirely.